It is well known for lubricating oils to contain a number of surface active additives (including antiwear agents, dispersants, or detergents) used to protect internal combustion engines from corrosion, wear, soot deposits and acid build up. Often, such surface active additives can have harmful effects on engine component wear (in both iron and aluminium based components), bearing corrosion or fuel economy. A common antiwear additive for engine lubricating oils is zinc dialkyldithiophosphate (ZDDP). It is believed that ZDDP antiwear additives protect the engine by forming a protective film on metal surfaces. ZDDP may also have a detrimental impact on fuel economy and efficiency and copper corrosion. Consequently, engine lubricants may also contain a friction modifier to obviate the detrimental impact of ZDDP on fuel economy and corrosion inhibitors to obviate the detrimental impact of ZDDP on copper corrosion. Other additives may also increase lead corrosion.
Further, engine lubricants containing phosphorus compounds and sulphur have been shown to contribute in part to particulate emissions and emissions of other pollutants. In addition, sulphur and phosphorus tend to poison the catalysts used in catalytic converters, resulting in a reduction in performance of said catalysts.
With increasing control of both the formation of sulphated ash and release of emissions (typically to reduce NOx formation, SOx formation) there is a desire towards reduced amounts of sulphur, phosphorus and sulphated ash in engine oils. Consequently, the amounts of phosphorus-containing antiwear agents such as ZDDP, overbased detergents such as calcium or magnesium sulphonates and phenates have been reduced. As a consequence, ashless additives such as esters of polyhydric alcohols or hydroxyl containing acids including glycerol monooleate and alkoxylated amines have been contemplated to provide friction performance. However there have been observations that ashless friction modifiers may in some instances increase corrosion of metal, namely, copper or lead. Copper and lead corrosion may be from bearings and other metal engine components derived from alloys using copper or lead. Consequently, there is a need to reduce the amount of corrosion caused by ashless additives. However, reducing the levels of antiwear and other ash-containing additives may result in increasing amounts of wear and/or copper corrosion.
Canadian Patent CA 1 183 125 (by Barrer, filed Sep. 10, 1981) discloses lubricants for gasoline engines containing alkyl-ester tartrates, where the sum of carbon atoms on the alkyl groups is at least 8. The tartrates are disclosed as antiwear agents. Other references disclosing tartrates and/or tartrimides include International Publication WO 2006/044411, and US patent applications for internal combustion engines requiring reduced amounts of sulphur, sulphated ash, and phosphorus. The lubricant composition has antiwear or anti-fatigue properties. The lubricating compositions are suitable for road vehicles.
U.S. Pat. No. 4,237,022 (by Barrer, filed Dec. 2, 1980) discloses tartrimides useful as additives in lubricants and fuels for effective reduction in squeal and friction as well as improvement in fuel economy.
U.S. Pat. No. 5,338,470 (by Hiebert, filed Dec. 10, 1992) and International Publication WO 2005/087904 (by Migdal, filed Mar. 11, 2004) disclose lubricants containing at least one hydroxycarboxylic acid ester or hydroxy polycarboxylic acid (in particular citrates or ethyl glycolate). The lubricant composition has anti-wear or anti-fatigue properties.
International Application WO2008/070307 (by Brown, filed Oct. 22, 2007) discloses engine lubricants containing antiwear agents based on malonate esters.
U.S. Pat. No. 4,436,640 (by Yamaguchi and filed on May 27, 1982) discloses a lubricant antiwear agent prepared by a two step reaction involving (i) reacting glycolic acid with an alcohol containing 1 to 6 carbon atoms, and (ii) reacting the product of (i) with phosphorus pentasulphide. The antiwear agent is reported to be useful for a cam-follower set.
Lubricants additives derived from thioglycolic acid derivatives have been contemplated as additives. Additives from thioglycolic acid derivatives are summarised in a variety of U.S. patents, Japanese patent application and an East German Patent. The U.S. patents include U.S. Pat. No. 4,157,970 (by Yaffe and filed on Jun. 12, 1979), U.S. Pat. No. 4,863,622 (by Chiu, filed Sep. 5, 1989), U.S. Pat. No. 5,132,034 (by Hsu, filed on Jul. 21, 1992 and June 1), U.S. Pat. No. 5,215,549 (by Hsu, filed on Jun. 1, 1993), and U.S. Pat. No. 6,127,327 (by Camenzind and filed on Jun. 24, 1999). The Japanese Patent Applications include 2005139238 A (by Yanagi and filed on Jun. 2, 2005), Japanese Patent Applications 10183161A (by Imai and filed on Jul. 14, 1998) and 10130679A (by Endo and filed on May 19, 1998), 05117680A (by Sato and filed on May 15, 1993). The East German Patent is DD 299533 A5 (by Buechner, published Apr. 23, 1992).